1. Field of the Invention
The present invention relates to a booster circuit of an electronic instrument, in particular, an electronic instrument in which an input voltage is less than a starting voltage of a general booster circuit.
2. Description of the Related Art
When a voltage of power supplied to an electronic instrument by a power supply is lower than that of a load circuit carrying out a desired function, it is necessary to convert, by using a booster circuit, the power from the power supply into boosted power of a voltage at which the load circuit can be operated, and to operate the load circuit by use of the boosted power. However, in recent years, power supplies in each of which the voltage of the supplied power is less than the starting voltage of the booster circuit have been increased as a result of consideration of downsizing, weight reduction, cost reduction, a design, and convenience. Accordingly, the power from the power supply as described above is becoming increasingly unusable for the operation of the load circuit.
For example, a fuel cell or solar cell of a single-cell type applies to the above-described condition. In this case, the voltage of the supplied power becomes 0.5 V to 0.9 V in the fuel cell and 0.3 V to 0.7 V in the solar cell, and the booster circuit cannot be started at such voltages. Accordingly, the power of these batteries is unusable for the operation of the load circuit.
Further, in the case where a capacitor such as an electric double layer capacitor has been discharged to a voltage less than the starting voltage of the booster circuit, the rest of power is unusable for the operation of the load circuit.
Still further, considering the above description, a thermoelectric element using the Seebeck effect is supplied with power of a voltage at which a usual load circuit cannot be started, and accordingly, the generated power is unusable for the operation of the load circuit.
Hence, if the starting voltage of the booster circuit can be lowered, the power from the power supply in the case where the voltage of the supplied power is low becomes usable for the operation of the load circuit. However, usually, in order to lower the starting voltage of the booster circuit, it is necessary to lower a threshold voltage of a driver transistor or the like in the booster circuit. In this case, an off-leak current of the driver transistor or the like is increased, and boosting efficiency is deteriorated owing to a power loss caused by the off-leak current. Specifically, in the case of using the power as described above, utilization efficiency thereof for the operation of the load circuit is lowered.
In this connection, an electronic instrument to be described below has been devised. If a configuration of the conventional electronic instrument is adopted, the starting voltage of the booster circuit can be lowered while hardly lowering the boosting efficiency of the booster circuit.
FIG. 6 shows a schematic circuit block diagram of the conventional electronic instrument having the feature described above.
As shown in FIG. 6, the conventional electronic instrument (for example, refer to JP 11-18419 A (FIG. 2)) includes: a power supply 101 in which the voltage of the supplied power is less than the starting voltage of the usual booster circuit; a first booster circuit 102 in which the boosting efficiency is low at a low starting voltage; a second booster circuit 103 in which the boosting efficiency is high at a high starting voltage; a capacitor 105; and a load circuit 104 carrying out a desired function. An output terminal 121 of the power supply 101 is connected individually to an input terminal 122 of the first booster circuit 102, an input terminal 125 of the second booster circuit 103, and a power supply terminal 123 of the first booster circuit 102. An output terminal 124 of the first booster circuit 102 is connected individually to one of electrodes of the capacitor 105 in which the other electrode is connected to a GND terminal 108, a power supply terminal 126 of the second booster circuit 103, an output terminal 127 of the second booster circuit 103, and a power supply terminal 128 of the load circuit 104. GND input terminals of the respective circuits and the power supply 101 are connected to the GND terminal 108. The conventional electronic instrument is configured as described above.
With the configuration described above, even if a voltage of power supplied from the power supply 101 is less than a starting voltage of the second booster circuit 103, and if the voltage concerned is equal to or more than a starting voltage of the first booster circuit 102, then the first booster circuit 102 is started by use of the power from the power supply 101, and the power from the power supply 101 can be converted into first boosted power of a voltage higher than the voltage of the power concerned and equal to or more than the starting voltage of the second booster circuit 103. In addition, the second booster circuit 103 is started by use of the first boosted power, and the power from the power supply 101 can be converted into second boosted power of a voltage higher than the voltage of the power concerned and equal to or more than an operating voltage of the load circuit 104. Accordingly, the load circuit 104 can be driven by use of the second boosted power.
Specifically, in the conventional electronic instrument described above, the second booster circuit in which the boosting efficiency is high at the high starting voltage is started by use of the first boosted power obtained by converting the power from the power supply by the first booster circuit in which the boosting efficiency is low at the low starting voltage. Then, after the second booster circuit is started, the load circuit is operated by use of the second boosted power obtained by converting the power from the power supply by the second booster circuit. Accordingly, even the power from the power supply supplying the power of the low voltage is efficiently usable for the operation of the load circuit.
The conventional electronic instrument with the configuration described above has a configuration in which the respective components such as the power supply terminal of the second booster circuit and the load circuit are connected to the output terminal of the first booster circuit. Accordingly, the first boosted power is not only consumed for charging the above-described capacitor but also consumed by the respective components described above. Note that although not being described for the configuration of the conventional electronic instrument, it has also been devised to stop the second booster circuit and the like as the respective components when the capacitor is charged with the first boosted power. However, circuit scales and the like of the respective components are large, and accordingly, power consumption thereof is not small even if the components concerned are stopped.
Hence, in the conventional electronic instrument with the configuration described above, when the first boosted power outputted by the first boosting circuit falls down to less than the power consumption of the respective components, the voltage of the capacitor does not rise to equal to or more than the voltage at which the second booster circuit can be started. Accordingly, the second booster circuit cannot be started. Therefore, it has been necessary to set a capability of the first booster circuit at a capability of outputting the first boosted power exceeding the power consumption of the respective components. Specifically, in the conventional electronic instrument with the configuration described above, it is necessary for the first booster circuit to have the capability exceeding current consumption of the components connected to the output terminal of the first booster circuit. In the general booster circuit, the higher the boosting capability is, the more an installation area and cost of the booster circuit are increased. Accordingly, the installation area and cost of the first booster circuit cannot be reduced, and as a result, there has been a problem in that the installation area and cost of the conventional electronic instrument cannot be reduced, either.
Further, in the conventional electronic instrument with the configuration described above, a charge speed for the capacitor is slowed down by the amount of power consumption of the respective components. As a result, it takes long from the supply from the power supply to the start of the second booster circuit.
Specifically, in the conventional electronic instrument with the configuration described above, there has been another problem in that it takes long from the supply of the power from the power supply to the operation of the load circuit by use of the power concerned.
According to a first means of the present invention, there is provided an electronic instrument, characterized by including: a power supply for supplying power; a first booster circuit that is started with the power from the power supply; a capacitor for storing power of the first booster circuit; a second booster circuit that is started with the power in the capacitor; a load circuit operating with power of the second booster circuit; a voltage detection circuit for detecting a voltage of the capacitor; and a switching element controlled by a voltage detection signal, in which, when determining that the voltage of the capacitor is a predetermined voltage or more, the voltage detection circuit turns off the switching element and starts the second booster circuit with the power of the capacitor.
With such a configuration as described above, a capability of the first booster circuit can be reduced, and accordingly, an actual measurement area and cost of the first booster circuit can be reduced. As a result, an installation area and cost of the electronic instrument can be reduced. Further, when the capability of the first booster circuit is not lowered, a speed for charging the capacitor is increased. Accordingly, the start of the second booster circuit can be accelerated. Therefore, the time from the supply of the power from the power supply to the use of the power for the operation of the load circuit can be shortened.
Further, according to a second means of the present invention, in addition to the first means, there is provided an electronic instrument characterized in that the switching element is on until the starting of the second booster circuit ends when it is determined that the capacitor has a predetermined voltage or more.
With such a configuration as described above, the second booster circuit can be started more reliably, and accordingly, the power from the power supply can be reliably used for the operation of the load circuit.
Still further, according to a third means of the present invention, in addition to the first means, there is provided an electronic instrument is characterized in that the first booster circuit adopts a boosting mode using a capacitor.
With such a configuration as described above, the first booster circuit can assemble the respective circuits and the capacitor in the same chip. Accordingly, the installation area and cost of the first booster circuit can be further reduced, and as a result, the installation area and cost of the electronic instrument can be further reduced.
As described above, at the time of charging the capacitor with the first boosted power of the first booster circuit, the electronic instrument of the present invention can separate the capacitor from the second booster circuit and the load circuit by means of the switching element. Accordingly, in comparison with the conventional electronic instrument in which the switching element is not provided, it is not necessary to set the capacity of the first booster circuit equal to or more than the capability for the power consumption of the second booster circuit and the load circuit, and the capability of the first booster circuit can be thus reduced to a great extent. When the capability of the first booster circuit is not lowered, the charge speed for the capacitor free from the power consumption of the second booster circuit and the load circuit can be improved. Accordingly, the time from the supply of the power from the power supply to the start of the second booster circuit can be shortened. Hence, the electronic instrument of the present invention can solve the problems inherent in the conventional electronic instrument in that it is impossible to reduce the installation area and cost of the first booster circuit because the capability thereof cannot be lowered, and in that it takes long from the supply of the power from the power supply to the operation of the load circuit by use of the power.